Radio altimeters are a type of equipment installed in vehicles capable of flight, such as fixed-wing aircraft or helicopters, to measure the altitude (altitude above ground level (AGL)) of such a vehicle above terrain therebelow while in flight. To do this, a radio altimeter uses the principle of radar, transmitting radio waves toward the ground and measuring a time delay between the transmission of radio waves and the reception of reflected radio waves.
Radio altimeters include radio altimeters based on a radar pulse limited altimetry scheme corresponding to an amplitude modulation scheme as well as to radio altimeters based on a frequency modulated-continuous wave (FM-CW) scheme corresponding to a frequency modulation scheme. Thereamong, the FM-CW radio altimeter will be described with reference to FIGS. 1 and 2.
FIG. 1 is a block diagram illustrating a configuration of a general FM-CW radio altimeter, while FIG. 2 is a view illustrating the principle of calculating altitude using the FM-CW scheme of FIG. 1.
When the FM-CW radio altimeter transmits a transmission signal by linearly changing a frequency thereof over time, a signal reflected from a target and returned to the FM-CW radio altimeter has a time delay corresponding to altitude, and here, a beat frequency between the transmission and reception signals is measured to calculate a distance.
An operation of the FM-CW radio altimeter will be briefly described hereinafter with reference to FIGS. 1 and 2.
A frequency of a signal generated by a waveform generator 300 is linearly varied by a voltage controlled oscillator (VCO) 305. The linearly varied frequency signal is divided by a directional coupler 310 so as to be input to a power amplifier 315 and a mixer 335, and a power-amplified signal is transmitted to the ground through an RF transmission antenna 320.
When a linearly varied frequency signal (RF signal) reflected from the ground is input through an RF reception antenna 325, the RF pulse signal is amplified by a low noise amplifier 330 and is subsequently input to the mixer 335. The mixer 335 mixes the local signal input from the directional coupler 310 and the RF signal input from the low noise amplifier 330 and outputs a beat frequency signal corresponding to a difference between the two signals.
The beat frequency passes through a band pass filter 340 to only filter out a signal within a predetermined bandwidth, and the filtered signal being amplified by an intermediate frequency (IF) amplifier 345 and subsequently input to a beat frequency discriminator 350. The beat frequency discriminator 350 identifies a beat frequency Fb of the received signal and calculates altitude based on the identified beat frequency.
Here, altitude may also be obtained by configuring a closed loop uniformly maintaining the beat frequency that appears in proportion to altitude and measuring frequency variations over time. Employing such a method, a bandwidth of an IF amp can be limited to be narrow, considerably increasing an S/N radio, and thus, a high level of precision in distance measurement may be obtained, even at a small transmission output.
Meanwhile, even in the case that a transmission output is increased, a detection distance of a target is limited due to a signal component directly leaked from a transmitting end to a receiving end; thus, in general, this method is largely used in short-range sensors for a short detection distance with a low transmission power.
The method of calculating altitude using a beat frequency (Fb) over distance is as shown in Equation (1). Here, as illustrated in FIG. 2, Tm denotes a period of the linearly varied frequency signal and ΔF denotes frequency variations of the linearly varied frequency signal.
                                          F            b                    =                                                                      2                  ·                  Δ                                ⁢                                                                  ⁢                                  f                  ·                                      T                    d                                                                              T                m                                      =                                                                                4                    ·                    Δ                                    ⁢                                                                          ⁢                                      f                    ·                                          f                      m                                                                                        3                  ·                                      10                    8                                                              ·              H                                      ⁢                                  ⁢                  where          ,                                          ⁢                                    T              m                        =                                                            1                                      f                    m                                                  ·                                  T                  d                                            =                                                                    2                    ⁢                    H                                    C                                =                                                                                                    2                        ⁢                        H                                                                    3                        ·                                                  10                          8                                                                                      ·                                          F                      b                                                        >                                      f                    m                                                                                                          (        1        )            
However, related art radio altimeters are generally classified as short-range or medium and long-range radio altimeters, in terms of the intended purpose thereof, and when a wide range of altitude measurement from a short distance to a long distance is required, the following problems may arise in the use thereof.
For example, a case in which a wide range of altitude measurement from 1 m to 10,000 m is required is taken as an example.
In the case of the FM-CW radar scheme, when an altitude is changed 10,000 times from 1 m to 10,000 m, the beat frequency Fb is also changed by 10,000 times, and here, it is considerably difficult to accurately measure a frequency range of such a degree and a bandwidth of a receiving end should be considerably widened when actually implemented, resultantly increasing a medium detectable signal (MDS) level of the receiver, which makes it difficult to measure a small reception signal when input. In particular, in a case in which the beat frequency Fb according to altitude needs to be uniformly maintained through feedback, frequency variations Δf (or fm) need to be changed with a difference of 10,000 times, and this is also substantially impossible to implement in actuality.
Thus, the pulse limited altimetry scheme is only used for medium and long-range detection and the FM-CW scheme is only used for short-range detection rather than for extremely short range detection.